Ch 1 · Particles in the atom and atomic radius · AS & A Level Chemistry 9701
Topic 1 of 4 in Atomic structure — AS & A Level Chemistry 9701 — 58 lessons.
Introduction
Every chemical property ultimately traces back to the structure of the atom: its central nucleus, the sub-atomic particles it contains, and the electrons arranged around it. This lesson establishes the quantitative description of atomic structure — the charges and masses of protons, neutrons and electrons — and uses it to explain how atomic and ionic radius change across the Periodic Table.
Key ideas — The nuclear atom
An atom consists of a small, extremely dense nucleus containing protons and neutrons, surrounded by a much larger volume of mostly empty space through which electrons move in shells. Almost all of the atom's mass is concentrated in the nucleus, yet the nucleus occupies only about 1/10 000 of the atom's overall diameter. The electrons, held in shells by electrostatic attraction to the positively charged nucleus, occupy the remaining volume but contribute negligibly to the total mass.
🧑🏫 Teacher's explanation
Imagine a massive football stadium where a single grain of sand sits exactly in the center of the pitch. That grain of sand is the nucleus, and the rest of the stadium is the atom. Even though the nucleus contains almost all the mass, it occupies a tiny fraction of the volume. The rest of the stadium is mostly empty space where electrons 'swarm' around.
📖 Definitions
🔍 Worked example — Easy
An ion has a mass number of 23 and an atomic number of 11, with a charge of +1. Determine the number of protons, neutrons, and electrons.
🔍 Worked example — Standard
Calculate the number of protons, neutrons, and electrons in a ^35_17Cl^- ion.
🔍 Worked example — Exam-style
A beam of particles is passed through an electric field between two parallel plates (positive plate and negative plate). If the particles are protons, neutrons, and electrons moving at the same velocity, describe the path of each.
⚠️ Common errors
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🎯 Exam question
Key ideas — Relative charges and masses of sub-atomic particles
Protons and neutrons (collectively nucleons) reside in the nucleus and have approximately equal mass, roughly 1836 times that of an electron, so the nucleus accounts for essentially all of an atom's mass. The proton's positive charge is equal in magnitude and opposite in sign to the electron's negative charge, so a neutral atom contains equal numbers of protons and electrons.
🧑🏫 Teacher's explanation
Imagine an atom is like a massive, busy football stadium. The nucleus is a tiny, heavy marble sitting right on the center circle of the pitch. Even though that marble is incredibly small compared to the whole stadium, it contains almost all the weight (mass) of the entire building. The electrons are like tiny, weightless gnats buzzing around the very top rows of the stands.
📖 Definitions
🔍 Worked example — Easy
An atom of Sodium has an atomic number of 11 and a mass number of 23. Calculate the number of protons, neutrons, and electrons in this neutral atom.
🔍 Worked example — Standard
Calculate the number of protons, neutrons, and electrons in a Magnesium ion with a charge of 2+ and a mass number of 24.
🔍 Worked example — Exam-style
An ion has 18 electrons, 20 neutrons, and a charge of 2-. Determine its atomic number and mass number.
⚠️ Common errors
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🎯 Exam question
Key ideas — Atomic number, mass number and particle counts
The atomic (proton) number, Z, is the number of protons in the nucleus of an atom, which determines the identity of the element. The mass (nucleon) number, A, is the total number of protons and neutrons in the nucleus. For a neutral atom, number of electrons = number of protons = Z, and number of neutrons = A - Z. For an ion of charge n, the number of electrons becomes Z - n (fewer electrons for a positive ion, more for a negative ion), while the numbers of protons and neutrons are unaffected by ionisation, since ionisation involves only the gain or loss of electrons.
🧑🏫 Teacher's explanation
Think of an atom like a busy airport terminal. The nucleus is the heavy, central security hub where most of the weight is concentrated, but it is incredibly tiny compared to the whole building. The electrons are like the travelers flying through the vast, empty terminal space around that hub. Even though travelers (electrons) are everywhere, they weigh almost nothing compared to the heavy luggage (protons and neutrons) sitting in the security hub.
📖 Definitions
🔍 Worked example — Easy
Calculate the number of protons, neutrons, and electrons in a neutral ^35_17Cl atom.
🔍 Worked example — Standard
Determine the subatomic particle counts for the magnesium ion, Mg^2+ (where A=24 and Z=12).
🔍 Worked example — Exam-style
An ion has 18 electrons, 20 neutrons, and a charge of -2. Determine its mass number A and its identity (element symbol).
⚠️ Common errors
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🎯 Exam question
Key ideas — Distribution of mass and charge; deflection in an electric field
Because nearly all the mass and all of the positive charge of an atom are concentrated in the tiny nucleus, while the negative charge is spread through the much larger surrounding volume as electrons, beams of protons, neutrons and electrons behave very differently in an electric field even when travelling at the same velocity. A beam of protons (positive, light) is deflected towards the negative plate. A beam of electrons (negative, very light) is deflected towards the positive plate, and, because electrons have such a small mass, this deflection is far greater than that of a proton beam of the same charge magnitude and speed. A beam of neutrons, being uncharged, experiences no force and passes through the field undeflected.
🧑🏫 Teacher's explanation
Imagine a massive, heavy bowling ball sitting in the middle of a large, empty gymnasium. Around the edges of the gym, tiny, lightweight ping-pong balls are zipping around. The bowling ball represents the nucleus—it holds almost all the weight (mass) but takes up almost no space. The ping-pong balls are the electrons, which are incredibly light but occupy the vast majority of the gym's volume.
📖 Definitions
🔍 Worked example — Easy
An ion has an atomic number of 11 and a mass number of 23. Calculate the number of protons, neutrons, and electrons in the Na^+ ion.
🔍 Worked example — Standard
A beam of particles is passed through an electric field. The particles are deflected towards the positive plate. Identify the particle and explain why it deflects more than a proton beam of the same velocity.
🔍 Worked example — Exam-style
An isotope of an element has 26 protons and 30 neutrons. Determine its mass number and write its symbol using the correct notation.
⚠️ Common errors
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🎯 Exam question
Key ideas — Trends in atomic and ionic radius
Across a period, atomic radius decreases from left to right. Each successive element adds a proton to the nucleus and an electron to the same outer shell; the increasing nuclear charge attracts the outer electrons more strongly without a corresponding increase in shielding, so the outer shell is pulled inwards and the atomic radius decreases.
🧑🏫 Teacher's explanation
Think of an atom like a magnet surrounded by a cloud of metal filings. The nucleus is the magnet, and the electrons are the filings. The 'radius' is essentially how far the outer filings are being pulled toward the center.
📖 Definitions
🔍 Worked example — Easy
Explain why the atomic radius of Magnesium (Mg) is smaller than the atomic radius of Sodium (Na).
🔍 Worked example — Standard
Compare the radius of the O^2- ion to the radius of the O atom.
🔍 Worked example — Exam-style
Predict the trend in ionic radius for the Group 2 elements (Mg^2+, Ca^2+, Sr^2+) and justify your answer.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
Question. An atom of an element X has the notation ^35_17X. It forms an ion X^-. (a) State the number of protons, neutrons and electrons in the ion X^-. [3] (b) Explain why the ionic radius of X^- is larger than the atomic radius of X. [2]
🔍 Worked example — Easy
An atom of Magnesium has the notation ^24_12Mg. Determine the number of protons, neutrons, and electrons in a Mg^2+ ion.
🔍 Worked example — Standard
Calculate the number of neutrons in an isotope of Chlorine with a mass number of 37.
🔍 Worked example — Exam-style
An ion has 18 electrons, 18 neutrons, and 17 protons. Write its notation in the form ^A_ZX^n and identify the element.
⚠️ Common errors
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Practice
Answers